GEMOC ARC National Key Centre

Bruce W. Chappell

GEMOC ANU

The restite model not only contends that many granites contain crystals of unmelted but magmatically equilibrated source material, or restite, but also that variations in the proportion of such restite, and melt, accounts for much of the variation in many granite suites.

The restite model had its origin in a study of what are now called I-type granites, in the Moonbi district of the New England Batholith of south-eastern Australia (Chappell, 1966). The concept was suggested, first, by the extraordinary linear trends of some elements and the lack of any curved trends among those granites. Also, the granites had textural features such as clusters of mafic minerals and uniform plagioclase cores that were interpreted as resulting from the incorporation of partly recrystallised source material. At that time, mafic enclaves were assigned an important role as a source of individual crystals of restite. These features could perhaps have been interpreted just as satisfactorily using a model of magma mingling, but such processes have never appealed to this author because of physical difficulties, and the discovery within a few years of S-type granites which clearly evolved in the same way, despite a lack of igneous enclaves. What may have been largely a prejudice against magma mingling has now been confirmed as realistic by more detailed studies of patterns of chemical variation (Chappell, 1996a) and by the fact that suites in the Bega Batholith do not change systematically in isotopic composition with changes in chemical composition (Chappell and McCulloch, 1990). It must also be noted that there are enclaves in S-type granites that others have regarded as igneous, but which Doone Wyborn has pointed out have striking compositional similarities with calcareous beds in the Ordovician sediments of the Lachlan Fold Belt, and which are probably lithic enclaves from such a source, but perhaps from older rocks.

Conceived in New England, the restite model grew to maturity in the Lachlan, where most of the more mafic I-type granites show similar distinguishing features, with the additional and striking supporting evidence from the mafic S-type granites. There have been three publications specifically on this subject, by White and Chappell (1977), Chappell et al. (1987), and Chappell and White (1991). A major fourth paper is in preparation (Chappell et al., 1999). Among the Lachlan granites, powerful support for the restite model comes from the contrast in petrological and compositional features with suites that evolved through fractional crystallisation, such as Boggy Plain (Wyborn, 1983), and the inability of that other process to account for many of the compositional trends in both I- and S-type suites. The universal occurrence of age inheritance in zircon cores that are surrounded by rims yielding the magmatic age (e.g. Williams, 1995), in those suites for which on other grounds restite would be assigned an important role, has been a critical observation. This not only shows that the magmas were never completely molten, but also, the presence of those old zircons implies that zircon was always saturated in the melts involved in producing those suites, which is confirmed by the patterns of bulk rock Zr variation. Since those melts were always saturated in zircon, they could not have been at temperatures above the zircon saturation temperature of Watson and Harrison (1984) for any significant time. The calculated zircon saturation temperature for melts with compositions corresponding to the most mafic rocks of these suites are ~ 750 °C, very much lower than the temperatures that would have been required for material of that composition to have been completely or even largely molten. For the same reason, it is unlikely that the rocks are cumulates produced from melts of somewhat less mafic compositions, but that possibility can easily be ruled out on other grounds. These arguments imply that the melt involved in the production of these suites was both felsic, and at low temperature, and confirms that the more mafic rocks have that property because of the presence of crystals of entrained restite. Chappell et al. (1999) therefore distinguished between granites, both I- and S-type, which formed at low magmatic temperatures, and other I-type granites formed at high temperatures, such as the Boggy Plain Suite of Wyborn et al. (1987). Under that scenario, most granites of the Lachlan Fold Belt, and also the Moonbi Granites of New England, formed by partial melting of quartzofeldspathic rocks in the crust. In most cases, the melt compositions were close to those determined experimentally by Tuttle and Bowen (1958) at the lowest magmatic temperatures, and variation within the suite, except at very felsic compositions, is the result of fractionation of restite from melt. In some cases, the felsic melts evolved further by fractional crystallisation of quartz and feldspars, after separation of restite, to produce rocks with highly fractionated trace element abundances. Less often, the initial melting continued to higher temperatures, but the more mafic magmas when emplaced nevertheless contained crystals of restite, including zircon. In such cases, the more mafic compositions evolved through restite crystal fractionation, and the more felsic sometimes also by fractional crystallisation, e.g. in the S-type Koetong Suite (Chappell and White, 1998), again at times leading to extremes in trace element compositions.

The variation diagram for Zr in the S-type Bullenbalong Suite published by White and Chappell (1988) showed Zr contents increasing with decreasing total FeO contents (increasing SiO2), with a sharp inflexion at close to 3.5% FeO (68% SiO2), leading to progressively decreasing Zr abundances as the rocks became more felsic. Such a kink in Zr abundances is characteristic of rock series formed by fractional crystallisation, such as the Boggy Plain pluton (Wyborn, 1983) where Zr is not saturated in the early formed more mafic rocks, so that its abundance increases in the melt, including trapped inter-cumulus melt. The combination of falling temperatures and increasing Zr concentrations in the melt leads to saturation in that element, following which Zr abundances decrease in subsequently formed rocks. However, the variation within the Bullenbalong Suite cannot be accounted for in that way, because all mafic S-type granites that have been examined, including those of that suite, contain abundant age inheritance in the zircon crystals, which implies that the silicate melts involved in the production of that suite were always saturated in Zr. For that reason, the inflexion in Zr abundances cannot be due to the development of Zr saturation during fractional crystallisation and some other mechanism is required. It is proposed that the more mafic Bullenbalong granites have retained specific sedimentary source rock compositions without fractionation, and that the abundances of Zr and SiO2 were positively correlated in those source rocks. Only the most SiO2- and Zr-rich source rocks produced magmas in which the melt could and did fractionate from the restite, to produce the more felsic part of the variation in the suite, in which those two components are negatively correlated. In this model, the more mafic granites of the Bullenbalong Suite represent source sedimentary rocks that were partially melted to the magmatic stage, following which they moved upwards (to sometimes erupt) and solidified without any fractionation of melt from restite. This is a significant modification, but an enhancement, of the restite model. Some current data hint at a similar situation for more mafic I-type granites.

The restite model has not been without its critics, most notably Wall et al. (1987). Those authors proposed that it be left to Rest In Peace. Perhaps a more reasonable view, based on abundant field, petrographic, geochemical, experimental, and zircon age inheritance data, is that it represents a Revolution In Petrogenesis. However, the significance of the restite model is not restricted to petrogenetic aspects, and there are wider implications, discussed in some detail by Chappell (1996b). It alleviates the problems of introducing large amounts of heat into the crust, since the requirement is for a partially molten magma rather than a complete melt, and at substantially lower temperatures. It has important implications for how the crust has been fractionated into a more mafic lower crust and granodioritic upper crust compositions, and has indeed been a factor in determining the composition of the exposed continental crust. However, probably the two most important implications are that it provides the opportunity to better estimate the composition of the source rocks in the deeper crust, and its implications for the production of mineral deposits. The first of those, means that many granites can be used as compositional probes of the deep crust, and its first use in that way was to relate compositional properties of granites to corresponding features in their source rocks, leading to the I- and S-types (Chappell and White, 1974, 1992). Later, an "I-S line" was recognised in the eastern Lachlan Fold Belt and correlated with an eastern limit of thick metasedimentary crust in that belt by White et al. (1976). Subsequently, Chappell et al. (1988) extended that principle to the whole of that belt in their proposition that the granite provinces that can be recognised, correspond with basement terranes. The implications of the restite model and the recognition of low-and high-temperature I-type granites by Chappell et al. (1999), for the formation of mineral deposits, are clear and obvious. To quote from that paper: "Because of both their higher temperatures, and a greater potential to undergo changes in composition, including an increase in the activity of H2O, through the process of fractional crystallisation, the high-temperature granite types are more likely to be related to significant mineralisation. This is clearly seen in eastern Australia (Blevin and Chappell, 1992) where, for example, most of the Devonian- and Carboniferous-age I-type granites of the Lachlan Fold Belt, largely of low-temperature origin, are conspicuously lacking in associated mineralisation. The better understanding of these two granite types that we are now developing, along with better criteria to recognise them, should have important implications for mineral exploration."

REFERENCES

Blevin, P L, and Chappell, B W, 1992. The role of magma sources, oxidation states and fractionation in determining the granite metallogeny of eastern Australia. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 305-316.

Chappell, B W, 1966. Petrogenesis of the granites at Moonbi, New South Wales. PhD thesis, Australian National University, Canberra (unpubl.).

Chappell, B W, 1996a. Magma mixing and the production of compositional variation within granite suites: evidence from the granites of southeastern Australia. Journal of Petrology 37, 449-470.

Chappell, B W, 1996b. Compositional variation within granite suites of the Lachlan Fold Belt: its causes and implications for the physical state of granite magma. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 159-170.

Chappell, B W, and White, A J R, 1974. Two contrasting granite types. Pacific Geology 8, 173-174.

Chappell, B W, and White, A J R, 1992. I- and S-type granites in the Lachlan Fold Belt. Transactions of the Royal Society of Edinburgh: Earth Sciences 83, 1-26.

Chappell, B W, White, A J R, and Wyborn, D, 1987. The importance of residual source material (restite) in granite petrogenesis. Journal of Petrology 28, 1111-1138.

Chappell, B W, Bryant, C J, Wyborn, D, White, A J R, and Williams, I S, 1999. High- and low-temperature I-type granites. Resource Geology (in press).

Chappell, B W, and McCulloch, M T, 1990. Possible mixed source rocks in the Bega Batholith: constraints provided by combined chemical and isotopic studies. Abstracts Geological Society of Australia 27, 17.

Chappell, B W, and White, A J R, 1991. Restite enclaves and the restite model. In Didier, J. and Barbarin, B. (eds), Enclaves and Granite Petrology, pp. 375-381. Elsevier, Amsterdam.

Chappell, B W, and White, A J R, 1998. Development of P-rich granites by sequential restite fractionation and fractional crystallisation: the Koetong Suite in the Lachlan Fold Belt. Acta Universitatis Carolinae, Geologica 42, 23-27.

Chappell, B W, White, A J R, Blundy, J D, Williams, I S, Johnnies, W, and Wyborn, D, 1999. Crystal fractionation of entrained restite: its role in producing compositional variation in some granites and related volcanic rocks, and implications. (in preparation).

Chappell, B W, White, A J R, and Hine, R, 1988. Granite provinces and basement terranes in the Lachlan Fold Belt, southeastern Australia. Australian Journal of Earth Sciences 35, 505-521.

Tuttle, O F, and Bowen, N L, 1958. Origin of granite in the light of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O. The Geological Society of America Memoir 74.

Wall, V J, Clemens, J D, and Clarke, D B, 1987. Models for granitoid evolution and source compositions. The Journal of Geology 95, 731-730.

Watson, E B, and Harrison, T M, 1983. Zircon saturation revisited: temperature and composition effects in a variety of crustal magma types. Earth and Planetary Science Letters, 64, 295-304.

White, A J R, and Chappell, B W, 1977. Ultrametamorphism and granitoid genesis. Tectonophysics 43, 7-22.

White, A J R, and Chappell, B W, 1988. Some supracrustal (S-type) granites of the Lachlan Fold Belt. Transactions of the Royal Society of Edinburgh: Earth Sciences 79, 169-181.

White, A J R, Williams, I S, and Chappell, B W, 1976. The Jindabyne Thrust and its tectonic, physiographic and petrogenetic significance. Journal of the Geological Society of Australia 23, 105-112.

Williams, I S, 1995. Zircon analysis by ion microprobe: the case of the eastern Australian granites. Leon T. Silver 70th Birthday Symposium and Celebration, The California Institute of Technology, 27-31.

Wyborn, D, 1983. Fractionation processes in the Boggy Plain zoned pluton. PhD thesis, Australian National University, Canberra (unpubl.).
Wyborn, D, Turner, B S, and Chappell, B.W., 1987. The Boggy Plain Supersuite: a distinctive belt of I-type igneous rocks of potential economic significance in the Lachlan Fold Belt. Australian Journal of Earth Science 34, 21-43.